In this chapter, we present multicast routing on the Internet. First, IP multicasting addressing discussed both for IPv4 and IPv6. Then, the role of Internet Group Management Protocol (IGMP) in IP multicasting is described. This is followed by presentation of several well-known intra-domain multicasting protocols, namely, Distance-Vector Multicast Routing Protocol (DVMRP), Multicast OSPF (MOSPF), Core Based Trees (CBT), and Protocol Independent Multicast (PIM). Finally, inter-domain multicasting was discussed.